This invention relates generally to gas chromatography methods and apparatus, and more specifically relates to the flame ionization detectors which may be utilized with gas chromatography systems.
Among the most common and effective devices currently used with gas chromatography systems for detecting and measuring the separated components in the effluent gas, are the so-called flame ionization detectors (FID). These detectors basically consist of a diffusion-type hydrogen burner, so arranged that the flame is burned between two electrodes, with a typical potential difference of the order of 100-300 volts being maintained between the electrodes. The effluent gas from the chromatography column is mixed with the hydrogen stream to the burner nozzle, and the resultant mixture is burned in air or oxygen. When the column effluent contains organic substances, these will ionize in the hydrogen flame of the detector, changing the conductivity of the flame, and consequently the intensity of the ion current. The ion current between the two electrodes, one of which is usually the burner nozzle itself, can be recorded after appropriate amplification.
A significant problem which in the past has been encountered in use of FID systems is one of assuring that the flame is actually operating. Relatively crude and unacceptable techniques have been utilized for such purposes.
Practitioners in the art, for example, have commonly sought to detect the flame's condition by such cumbersome and time-consuming techniques as inserting small bits of paper into the detector flame zone, removing same and observing the paper for charring. In a similar vein, a mirror has been intermittently positioned over the flame ionization detector for purposes of collecting water vapor -- the presence of same being taken as an indication of a burning flame. Where techniques such as these established that the flame was extinquished, re-ignition could be effected via an ignition coil positioned proximate the burner nozzle -- i.e. by passing a current through the coil to render same in a hot or glowing condition.
While it has been proposed, e.g. in U.S. Pat. No. 3,340,013, to minimize the flame-out problem by periodically re-igniting the burner, such approach is based upon use of overly complex and costly techniques, i.e. upon a programming system designed to effect heating of the ignition coil at the end of each chromatographic cycle -- regardless of the actual condition of the flame. An obvious correlative difficulty with this approach is that re-ignition may not be effected at the time it is actually needed.
In accordance with the foregoing, it may be regarded as an object of the present invention, to provide an improved flame ionization detector for use in gas chromatography systems, which includes simple, dependable, and highly effective means for indicating that the detector flame has become extinguished, to thereby enable manual or automatic re-ignition of such flame.
It is a further object of the invention, to provide an improved flame ionization detector of the aforementioned type, wherein the means for detecting flame extinction is combined with a re-ignition coil, with but a pair of electrical leads being required to enable both functions.
It is a yet further object of the present invention, to provide a flame ionization detector system of the aforementioned type, wherein flame polarization may be effected through the same lead pair utilized for the flame extinction detection function, and the flame re-ignition function.